1. Field of the Invention
This invention relates to processing techniques capable of enlarging the size of a spatially-sampled video image originally having predetermined horizontal and vertical resolutions and, more particularly, to processing techniques which preserve, in the enlarged video image, the sharpness of the edge information of the original video image.
2. Description of the Prior Art
Incorporated by reference herein is our earlier U.S. Pat. No. 5,355,328, issued Oct. 11, 1994, and entitled "Resampling Apparatus Suitable for Resizing a Video Image". This patent discloses both resampling apparatus for reducing the size of a spatially-sampled original video image in the horizontal and/or vertical directions and resampling apparatus for enlarging the size of a spatially-sampled original video image in the horizontal and/or vertical directions. The size-reduction case, which involves a decrease in the spatial sampling of the video information of the originally-sampled video image, does not adversely affect the appearance of a reduced-sized displayed image. However, the size-expansion case, which involves an increase in the spatial sampling of the video information of the originally-sampled video image, does adversely affect the appearance of an enlarged-sized displayed image. Specifically, by causing a smearing or blurring over an extended area in the appearance of the sharp edges that appear in an originally-sampled displayed image. The reason for this is that the data defined by each of the increased samples of the size-enlarged image is derived by interpolation of the image data samples in the originally-sampled image (since the originally-sampled image data is the only image data that is available).
More particularly, when it becomes necessary to enlarge an original spatially-sampled image or picture using digital processing techniques, where the original spatially-sampled image is already fixed in horizontal and vertical resolution by the original horizontal and vertical spatial-sampling periods, the enlarged image appears defocused or blurred because it does not contain the additional high-frequency image data necessary to support the new higher spatial-sampling structure. Traditional approaches to enlarging pictures include interpolating the image samples with a digital filter. The better the filter, that is, the closer to the ideal brick-wall filter response, the better the resultant image.
Unfortunately, even with the best interpolation filter, the image will never appear sharp for images that contain edge-like information. This is because enlargement spreads the image information over a wider spatial area, and the interpolation fills newly created sample positions in-between original samples as best as is possible. The resulting edge after interpolation is spread over a wider area, which causes the edge to lose fidelity and appear soft or blurred. The reason for this is that the enlarged image needs more information than is available to the interpolator. In fact, the enlarged image needs to have extra higher-frequency information added in order to reconstruct the edges similar to that before enlargement.
Added extra higher-frequency information (i.e., bandwidth extension) is also needed for the case in which it is desired to display a digital video image on a monitor having horizontal and vertical resolutions that are higher than the original horizontal and vertical resolutions of the digital video image to be displayed.
Traditional approaches to bandwidth extension include inverse filtering, where it is assumed that some lowpass filtering process was responsible for the blur, and that by applying the inverse of the filter, this blur can be removed. These techniques are difficult to implement, and are highly sensitive to noise. Other approaches include analyzing the image information in the frequency domain, and after determining the relative cosine and sine contributions extend the frequencies to higher order and add them into the image. This unfortunately is computationally very expensive and does not reliability give good subjective results. In fact, all such approaches fall short of the subjective improvement required.
Another approach serves to enhance enlarged images by boosting what energy already exists in the image signal to greater levels, thereby giving an overall subjectively looking sharper image appearance. This approach is accomplished by peaking, whereby the higher-frequencies in the image are boosted in amplitude relative to the lower frequencies. However, this does not create the additional bandwidth or frequencies needed to produce the edge profile defined in the original image. Therefore, a peaking approach shows improvement only for very small enlargement factors (which is usually well below an enlargement of two), and the performance of this peaking approach is further hampered by the quantity of noise existing in the image.
The synthetic bandwidth extension approach envisioned by the present invention is effective in substantially preventing the occurrence of any adverse affect in the appearance of an enlarged-sized displayed image of a sampled original image, in which the enlarged-sized displayed image has had its sampling frequency increased prior to display.